Method of making silethynyl polymers

ABSTRACT

A method of making silethynyl polymers having units of the general formula --(R 2  SiC.tbd.C)-- wherein each R independently denotes a hydrogen atom, an alkyl, aryl or alkenyl group or a substituted alkyl, aryl or alkenyl group having up to 18 carbon atoms comprises adding a lithium salt of one or more diethynylsilanes of the general formula R 2  Si(C.tbd.CH) 2  to one or more dihalosilanes of the general formula R 2  SiX 2  where X is a halogen atom and allowing the components to react. Copolymers with alternating units can be made. Linear polymers are made preferentially. Limiting the use of solvent increases the percentage of linear polymers produced.

This invention relates to a method of making silethynyl polymers,particularly copolymers with alternating units, and also of improvingthe yield of linear silethynyl polymers whether homopolymers orcopolymers are produced.

Linear silethynyl polymers have been known for some time and have beendescribed for example in G.B. Patent Specification 914 935. Thesepolymers have repeating units of the general formula --(Y₂ SiC.tbd.C)--,wherein Y may be a saturated, ethylenically unsaturated or aromatichydrocarbon group. The polymers of G.B. Specification 914 935 have beenprepared by reacting an organodifluoro silane with an alkali metalacetylide.

In G.B. Patent Application 2 204 041 a method is described for thepreparation of cyclic silethynyl polymers having at least 4 siliconatoms per molecule. This method comprises the reaction of a lithium saltof one or more diethynylsilanes with one or more dihalosilanes. In theexamples a dichlorosilane solution in tetrahydrofuran was slowly addedto a solution of the lithium salt of a diethynylsilane, producing mainlycyclic silethynyl polymers. When copolymers are made according to themethod of G.B. Patent Application 2 204 041 the different units of thepolymer are randomly distributed throughout the polymers.

We have now found that in the production of copolymers having an equalnumber of two different silethynyl units the reaction product has analternating sequence of said silethynyl units if the order of adding thereagents described in G.B. Patent Specification 2 204 041 is reversed.We have also found that reversing the addition order of reagents resultsin a higher yield of linear silethynyl polymers compared with the methodof the prior art.

According to the invention there is provided a method of makingsilethynyl polymers having units of the general formula --(R₂SiC.tbd.C)-- wherein each R independently denotes a hydrogen atom, analkyl, aryl or alkenyl group or a substituted alkyl, aryl or alkenylgroup having up to 18 carbon atoms, which comprises adding a lithiumsalt of one or more diethynylsilanes of the general formula R₂Si(C.tbd.CH)₂ to one or more dihalosilanes of the general formula R₂SiX₂, wherein R is as defined above and X is a halogen atom and allowingthe components to react.

The lithium salt of a diethynylsilane has the general formula R₂Si(C.tbd.CLi)₂ and can be prepared by reacting a diethynylsilane with analkyllithium compound for example butyl lithium. Such reaction iscarried out by mixing the ingredients and is preferably carried out inthe presence of a solvent comprising tetrahydrofuran, aromatichydrocarbon, aliphatic hydrocarbon or an ether solvent. Diethynylsilanesmay themselves be obtained for example by reacting a dihalosilane of thegeneral formula R₂ SiX₂ with HC.tbd.CMgCl in tetrahydrofuran. The Rsubstituent of the lithium salt may be hydrogen, alkyl for examplemethyl, ethyl, hexyl, dodecyl or octadecyl, aryl for example phenyl ornapthyl, alkenyl for example vinyl, allyl or hexenyl or substitutedgroups such as halogenated alkyl, tolyl or styryl. Preferably the Rsubstituents are alkyl or aryl, most preferably methyl or phenyl. Each Rsubstituent may be different or the same as the other R substituent.

The dihalosilane reactants are known substances, many of which arecommercially available. Preferably the dihalosilane is a dichlorosilane.The other substituents of the dihalosilane may be hydrogen, alkyl forexample methyl, ethyl, hexyl, dodecyl or octadecyl, aryl for examplephenyl or napthyl, alkenyl for example vinyl, allyl or hexenyl, orsubstituted groups such as halogenated alkyl, tolyl or styryl.Preferably these substituents are alkyl or aryl. The most preferreddihalosilanes are dimethyldichlorsilane, diphenyldichlosorilane andmethylphenyldichlorosilane.

The polymers produced by the method of the invention may be linear or amixture of cyclic and linear polymers. Cyclic polymers have the generalformula [R₂ SiC.tbd.C]_(n) wherein n is an integer. Linear polymers havethe average formula R"--[R₂ SiC.tbd.C]_(n) --R₂ SiR" wherein n is aninteger, R is as defined above and R" is selected from R, --C.tbd.CH,--OR, --X and --OH wherein X is halogen. This type of terminating unitsis produced depending on the reaction condition and/or solvents used.Preferably R" denotes --R or --C.tbd.CH.

The choice of diethynylsilanes and dihalosilanes will determine thesilethynyl polymers produced. The diethynylsilane which is used to formthe lithium salt may be represented by the general formula R⁰ R¹Si(C.tbd.CH)₂, the dihalosilane by the general formula R² R³ SiX₂. IfR⁰, R¹, R² and R³ are all the same or a combination of R⁰ and R¹ is thesame as the combination of R² and R³ the copolymers produced will behomopolymers. By choosing silanes in which the combination of R⁰ and R¹is different from the combination of R² and R³ or by choosing mixturesof two or more different diethynylsilanes and/or dihalosilanes,silethynyl polymers are produced which are copolymers having two or moredifferent units.

If, for example R⁰, R¹, R² and R³ are all methyl groups, mainly lineardimethylsilethynyl polymers are produced. The reaction of the lithiumsalt of dimethyldiethynylsilane with methylphenyldichlorosilane,according to the invention, will result in a silethnyl polymer havingsome units with two methyl substituents on the silicon atom and someunits with a methyl and phenyl substituent on the silicon atom. Examplesof preferred copolymers are those wherein R⁰, R¹, R² and R³ arerespectively methyl, methyl, phenyl and phenyl, methyl, methyl methyland phenyl or methyl, phenyl, phenyl and phenyl. If a mixture of silanesis used. e.g. a mixture of diphenyldichlorosilane andphenylmethyldichlorosilane the addition of a lithium salt of e.g.dimethyldiethynylsilane would result in a terpolymer having some unitswith two methyl groups attached to a silicon atom, some with a methyland a phenyl group attached and some with two phenyl groups attached. Aperson skilled in the art will immediately understand that numerouscombinations of silethynyl units can be achieved in a copolymer by usingthe method of this invention.

Polymers are made by using as reagents stoichiometric amounts of adiethynylsilane and a dihalosilane, i.e. one diethynylsilane for everydihalosilane. If only one type of each silane is used the method of thisinvention will provide a copolymer having an alternating sequence ofunits and not a random distribution, as was the case in the prior art.This is true for both linear copolymers and cyclic copolymers produced.The invention accordingly provides a method of making silethynylcopolymers having alternating units of the general formulae --(R⁰ R¹SiC.tbd.C)-- and --(R² R³ SiC.tbd.C)-- wherein each R⁰, R¹, and R³independently denotes a hydrogen atom, an alkyl, aryl or alkenyl groupor a substituted alkyl, aryl or alkenyl group having up to 18 carbonatoms, provided that the combination R⁰ and R¹ is different from thecombination R² and R³ which comprises adding a lithium salt of adiethynylsilane of the general formula R⁰ R¹ Si(C.tbd.CH)₂ to adihalosilane of the general formula R² R³ SiX₂ wherein X is a halogenatom and allowing the component to react.

It is preferred that in the method of the invention the lithium salt ismade of those silanes which, having regard to the silicon-bondedsubstituents, are the strongest nucleophile. This means that if onewants to make a silethynyl copolymer having alternating units of theform --(R⁰ R¹ SiC.tbd.C)-- and units of the form --(R² R³ SiC.tbd.C)--one will choose to use the lithium salt of the silane which is thestrongest nucleophile, i.e. either R⁰ R¹ Si(C.tbd.CLi)₂ or R² R³Si(C.tbd.CLi)₂ whilst the weaker nucleophile of R⁰ R¹ SiX₂ or R² R³ SiX₂will be used as the halosilane. This means for example that in order tomake a silethynyl polymer in which R⁰ and R¹ are methyl groups and R²and R³ are phenyl groups, one will preferably add the lithium salt ofdimethyldiethynyl silane to diphenyldichlorosilane and not the lithiumsalt of diphenyldiethynylsilane to dimethyldichlorosilane.

When producing silethynyl polymers according to the method of thisinvention it is particularly important to control the order of addition.It is found to be necessary to add the lithium salt only gradually tothe dihalosilane. It is believed, although the applicant does not wishto be bound by this theory, that the gradual addition is necessary inorder to ensure that the dihalosilane should be present instoichiometric excess during the reaction. The stronger the nucleophiliccharacter of the lithium salt of the diethynylsilane the more the speedof addition can be increased. The speed of addition of the lithium saltdepends to some extent on the efficiency of agitation as it is preferredto maintain an excess of the dihalosilane at all times, even on amicroscale, i.e. in the immediate neighbourhood of the incoming lithiumsalt of the diethynylsilane. Addition speeds can be increased accordingto the efficiency of the dispersion of the incoming lithium salts. For asmall scale operation typically such addition speed will be in the orderof 0.01 ml/minute to 10 ml/minute, preferably 0.1 to 0.5 ml/minute.

Low molecular weight cyclic polymers are believed to be formed byrearrangement of the growing polymers, probably under the influence oflithium organosilicon species as rearrangement catalysts. By ensuringthe dihalosilanes are present in stoichiometric excess it is believedthat the formation of these lithium-organosilicon compounds, and thusthe rearrangement reaction, is slower than the formation of the lithiumhalogen salts. Only towards the end of the addition of the lithium saltof diethynylsilane will the excess of dihalosilanes be so small that asmall proportion of rearrangement will be possible. The propoundedtheory may explain why the method of the invention tends to produce alarger amount of linear silethynyl polymers than the prior art method.

The reaction is preferably carried out in the presence of some solvent.The solvent may be the same or different from the solvent used in theproduction of the lithium salt of the diethynyl silane. The reaction canadvantageously be carried out immediately after producing the lithiumsalts and the solvent added together with the lithium salt. Additionalsolvent may be added to the dihalosilane. We have further found that byreducing the amount of solvent used in the method of the invention abetter yield of linear polymers is obtained, regardless of thesubstituents on the silicon atoms.

The temperature at which the reaction is carried out is not critical.The reaction may be carried out at or below ambient temperature or atelevated temperatures. Preferably the reaction is performed at atemperature in the range from about 15° to 30° C. The method of theinvention can provide high yields even when the reaction is carried outat ambient temperature (about 20° C.). If desired the reaction time maybe reduced by the use of elevated temperatures.

If all R groups are methyl groups, mainly linear silethynyl polymers areobtained which have the general formula R'--[(CH₃)₂ SiC.tbd.C]_(n)--Si(CH₃)₂ R' wherein each R' is selected from --C.tbd.CH, --CH₃,--OCH₃, --X and --OH where X is a halogen atom and n is an integer. Ifnot all R groups are methyl groups a mixture of linear and cyclicmaterials are formed. The linear materials have the general formulaR"--[RH₂ SiC.tbd.C]_(n) --SiR₂ R" wherein R and n are as defined aboveand R" denotes --C.tbd.CH, OH, X, R or OR. Because performing the methodof the invention in a reduced amount of solvent increases the yield oflinear polymers, it is preferred to keep the amount of solvent used inthe method of the invention to a minimum if linear homopolymers orcopolymers are desired.

When the reaction is complete the polymers may be recovered from thereaction mixture, for example by precipitation in an alcohol such asmethanol followed by filtration or solvent evaporation. Linear polymersthus obtained tend to be high viscosity materials which are usuallysolids or waxy materials. They are relatively high molecular weightmaterials wherein the value of n can have any value, but is generallybetween 8 and 250, mostly around 40.

The invention provides in another of its aspects silethynyl polymers andsilethynyl copolymers which are produced according to the method of theinvention.

Silethynyl polymers obtained by the method of this invention arebelieved to have useful optical and electronic properties arising fromtheir electron-rich nature. The polymers may be used for example assemiconductor materials or in waveguide technology. Due to the presenceof the acetylenic unsaturation the polymers may also serve asintermediates for further reaction, for example addition reaction withcompounds having siliconbonded hydrogen atoms. The latter compounds maye.g. bear certain functional groups which would thus be linked to thepolymers produced by the method of the invention.

There now follow a number of examples in which all parts and percentagesare expressed by weight.

EXAMPLES

A 2.85 molar solution or butyllithium in hexane (86.9 mmol) was added toa solution of A₂ Si(C.tbd.CH)₂ (43.5 mmol) in 50ml of tetrahydrofurancooled to -78° C. The resulting solution was stirred at ambienttemperature for two hours and transferred to a dropping funnel. Thesolution of A₂ Si(CH.tbd.CLi)₂ was added at a rate of 0.1 to 0.5 ml perminute to a solution of Y₂ SiCl₂ (43.5 mmol) in 50 ml tetrahydrofuran atambient temperature and stirred for two hours. This solution was thenadded to 200 ml of a saturated solution of NH₄ Cl in water. The organicphase was separated, washed with brine and dried over Na₂ SO₄. Afterfiltration the solvent was evaporated to give a solid which was washedwith methanol, filtered and dried under vacuum, resulting in a 95% yieldof the theoretical value which is due to the inaccuracy in thestoichiometry of the added reagents. The resulting compounds werestudied and analysed by super-critical fluid chromatography and nuclearmagnetic resonance using C¹³ and Si²⁹ isotopes. The A and Y groups andthe characterisation of the resulting compounds are given in the Tablebelow where Me denotes a methyl group and Ph a phenyl group. Comparativeexperiments, denoted by a C in front of the Example number, wereperformed using the method of G.B. Specification 2 204 041 and theresults are also shown in the Table.

                  TABLE                                                           ______________________________________                                                                   % Linear                                                                              Molecular                                  Example  A        Y        By Volume                                                                             Weight                                     ______________________________________                                        1        Me       Me       >90*    N/A                                        C1       Me       Me        0*     Cyclic                                     2        Ph       Ph       85      4100                                       C2       Ph       Ph       60      3400                                       3.sup.α                                                                          Ph       Ph       98      2900                                       4        Me       Ph       60-77   4560                                       C4       Me       Ph       60      3600                                       5+       Ph/Me    Ph/Me    87      N/A                                        C5+      Ph/Me    Ph/Me    70      2600                                       ______________________________________                                         *estimated value                                                              .sup.α performed in the presence of a minimum amount of solvent onl     +one of each of the A and Y substituents was Ph the other Me             

It becomes clear that the method of the invention yields a higherpercentage of linear silethynyl polymers than the prior art method andthat reducing the amount of solvent used to a minimum increases thispercentage even more. The polymer of Example 4 had alternating unitsdi-Me silethynyl and di-Ph silethynyl.

That which is claimed is:
 1. A method of making silethynyl polymershaving units of the general formula --(R₂ SiC.tbd.C)-- wherein each R isindependently selected from the group consisting of hydrogen, alkylgroups having up to 18 carbon atoms, aryl groups having up to 18 carbonatoms alkenyl groups having up to 18 carbon atoms, substituted alkylgroups having up to 18 carbon atoms, substituted aryl groups having upto 18 carbon atoms, and substituted alkenyl groups having up to 18carbon atoms, which comprises adding a lithium salt of one or morediethynylsilanes of the general formula R₂ Si(C.tbd.CH)₂ to one or moredihalosilanes of the general formula R₂ SiX₂, where the lithium salt isadded to maintain a stoichiometric excess of the dihalosilane to thelithium salt, R is as defined above, and X is a halogen atom andallowing the components to react.
 2. A method according to claim 1wherein each R is independently selected from the group consisting ofalkyl groups having up to 18 carbon atoms and aryl groups having up to18 carbon atoms.
 3. A method according to claim 1 wherein each R isindependently selected from methyl and phenyl.
 4. A method of makingsilethynyl copolymers having alternating units of the general formulae--(R⁰ R¹ SiC.tbd.C)-- and --(R² R³ SiC.tbd.C)-- wherein each R⁰, R¹, R²and R³ independently denotes a group R as defined above provided thatthe combination R⁰ and R¹ is different from the combination R² and R³,which comprises adding a lithium salt of a diethynylsilane of thegeneral formula R⁰ R¹ Si(C.tbd.CH)₂ to a dihalosilane of the generalformula R² R³ SiX₂ wherein X is a halogen atom and allowing thecomponents to react.
 5. A method according to claim 4 wherein R⁰ and/orR¹ are a stronger nucleophile than R² and/or R³.
 6. A method accordingto claim 4 wherein each of R¹, R² and R³ is independently selected fromthe group consisting of alkyl groups having up to 18 carbon atoms andaryl groups having to 18 carbon atoms.
 7. A method according to claim 4wherein each of R⁰, R¹, R² and R³ independently denotes a methyl or aphenyl group.
 8. A method according to claim 1 wherein X denotes achlorine atom.
 9. A method according to claim 1 wherein the componentsare allowed to react in the presence of a solvent.
 10. A methodaccording to claim 1 wherein the components are allowed to react at atemperature of from 15° to 30° C.
 11. A method according to claim 4wherein X denotes a chlorine atom.
 12. A method according to claim 4wherein the components are allowed to react in the presence of asolvent.
 13. A method according to claim 4 wherein the components areallowed to react at a temperature of from 15° to 30° C.